1. Field of the Invention
The present invention relates to a supercharging pressure controller for an internal combustion engine that drives a rotational shaft or a compressor of a turbocharger with a motor to achieve desired supercharging pressure.
2. Description of the Related Art
In order to increase power output and to reduce fuel consumption, a vehicle such as an automobile is equipped with a turbocharger engine, which uses a compressor of a turbocharger to supercharge intake air suctioned into engine cylinders. The turbocharger uses emission energy of exhaust gas flowing out of the cylinders of the internal combustion engine to rotate a turbine and to drive a compressor mounted coaxially with the turbine. Thus, the turbocharger supercharges the intake air suctioned into the cylinders of the engine. The supercharging pressure rises slowly and actual supercharging pressure is low in a low rotation speed range of the engine. As a result, charging efficiency is decreased and improvement of engine output is insufficient.
For the purpose of resolving this disadvantage, a certain supercharging pressure controller for an internal combustion engine drives a rotational shaft of a turbocharger with an electric motor to achieve desired supercharging pressure. Another supercharging pressure controller for an internal combustion engine has an auxiliary compressor in an engine intake pipe in addition to a usual compressor of a turbocharger and drives the auxiliary compressor with a motor to achieve the desired supercharging pressure.
However, in the conventional supercharging pressure controller for the engine, the target supercharging pressure cannot be maintained if electric power (motor power) W supplied to the motor is turned off immediately at time A when the actual supercharging pressure reaches the target supercharging pressure as shown in a time chart of FIG. 9C. It is because there is a delay in increase of emission energy and turbine rotation speed decreases compared to the value at the time when the turbine is driven by the motor, causing a drop in the actual supercharging pressure.
Therefore, a further supercharging pressure controller for an internal combustion engine (for example, as described in JP-A-2004-169629) gradually reduces the motor current from present motor current (maximum motor current) finally to zero if the actual supercharging pressure exceeds the target supercharging pressure.
However, since the supercharging pressure controller described in JP-A-2004-169629 supplies the maximum motor current until the actual supercharging pressure exceeds the target supercharging pressure, the actual supercharging pressure overshoots the target supercharging pressure. This supercharging pressure controller executes motor current control based on a deviation between the actual supercharging pressure and the target supercharging pressure if the actual supercharging pressure becomes equal to or lower than the target supercharging pressure during processing of gradually reducing the motor current. In consequence, even if the motor current approaches zero once, the motor current increases again. Thus, the motor rotation speed decreases and increases, causing hunting. Further, the motor current cannot be turned off over a long period. Thus, the electric energy used for the motor current control is wasted, resulting in poor fuel economy.